Slow blowing fuse



Au 11, 1964 B. w. B-AUMBACH 3,144,534

SLOW BLOWING FUSE Filed Dec. 12, 1960 2 Sheets-Sheet 1 [Wm/IA 1964 B. w. BAUMBACH 3,144,534

SLOW BLOWING FUSE Filed Dec. 1'2, 1960 2 Sheets-Sheet 2 United States Patent 3,144,534 SLOW BLOWENG FUSE Bertram W. Baumbaeh, Arlington Heights, EL, assignor to Littelfuse Incorporated, Des Plaines, ilL, a corporation of Illinois Filed Dec. 12, 1960, Ser. No. 75,178 Claims (Cl. 200-123) .This invention relates to improvements in time lag fuses which are adapted to break a circuit with a time delay under a given low sustained overload and immediately under a given high overload condition.

Fuses of the type just described are well known. One common form for such a fuse includes a heat sink or reservoir member comprising a bar of copper or the like having a heater coil wound therearound. One end of the heater coil is connected to the heat reservoir member and the other end to an end cap at one end of a glass tube forming a fuse housing. A fuse filament is soldered to the heat reservoir member and extends to an end cap at the other end of the glass tube. A'coil spring under tension is connected between the heat reservoir member and the former end cap, the spring urging the heat reservoir member and the fuse filament apart. In the event of a given sustained low overload condition, the heat generated by the heater coil is gradually absorbed by the heat vreservoir member which finally heats up to a point which softens the solder connection between the heat reservoir member and the fuse filament. When this occurs, the spring separates the reservoir member from the fuse filament to openvthe circuit involved. In the event of a given high overload, the fuse filament disintegrates to open the circuit.

The time lag fuses heretofore made left much to be desired. For example, in the time lag fuse described above, the fuse filament is placed under tension by the coil spring and additional strains placed upon the fuse filament by Vibration or shock can break or otherwise damage the same. Moreover, the construction and arrangement of, and the means for supporting these parts were such that the low overload blowing characteristics of the fuses were not consistent, and high current arcs sometimes developed under high overload which resulted in physical destruction of the fuses, with resulting danger to adjacent personnel and equipment.

Among the objects of the present invention are to provide a time lag fuse of the general type above described which avoids the disadvantages referred to above. More specifically, an object of the invention is to provide a time lag fuse of the general type above described which is of a more simple and rugged construction than fuses of a similar type heretofore made. the invention is to provide a time lag fuse which can be mass produced with consistent low overload and high overload blowing characteristics, and wherein the generation of dangerous arcs are prevented.

A further object of the invention is to provide a fuse,

as just described wherein the fuse parts are rigidly supported within a glass tube in a manner Where the fuse parts can be readily aligned and positionedtherein, and further wherein strains on the high overload fuse filament due to spring forces, vibration or shock is minimized or completely eliminated. v p

A related object of the invention is to provide a method for mass producing fuses of the type above described Another object of which makes possible consistent low overload blowing characteristics.

In accordance with one feature of the present invention,

the fuse parts contained within the fuse housing are constructed and arranged so that they can be readily rigidly positioned within the fuse housing in a definite predetermined position under mass production assembly techniques. The blowing characteristics of a fuse are, in part a function of the position and arrangement of the parts of the fuse within the fuse housing since these factors affect the heating of the fuse parts. The low overload characteristics of a fuse are further affected by the volume or size of the fuse parts. Accordingly, another feature of the invention involves a method of making the solder joint holding together the spring biased portions of the fuse in such a way that the solder joint has a consistent volume of solder applied in a manner which is adapted to mass production assembly techniques. To this end, a slug of solder of predetermined size is press-fitted or otherwise anchored in a hole in one of the conductive parts (preferably the heat reservoir member) to be soldered before assembly of the fuse, the slug of solder projecting beyond the surface of the conductive part. The solder operation requires only the positioning of the other conductive part to be soldered against the projecting portion of thework anchored slug and the momentary application of heat thereto. Another feature of the invention involves the prevention of dangerous arcing within the fuse when the fuse filament thereof disintegrates under high overload conditions. This is a particularly serious problem where the fuse is used in circuits where short circuit currents are quite high. In such case, a high energy arc can develop between relatively massive conductive portions of the fuse positioned on opposite sides of the disintegrated fuse filament. The space between one of the fuse terminals and the heat reservoir member offers a-particularly good path for the development of such an are. As previously indicated, such an are readily develops such pressure and temperature conditions that the fuse explodes. It has been discovered that these serious arcing conditions can be eliminated by sealing off the portion of the fuse housing containing the fuse fiament by a plug of insulating material.

As previously indicated, it is desirable that the fuse filament be relieved of strain due to spring, shock and vibration forces. To this end, in accordance with another feature of the invention, the assembly of fuse parts within the fuse housing are supported as an integral unit independently of the fuse filament and so that the spring forces do not act upon the fuse filament. In one form of the invention, the spring portion of the fuse comprises a compressed coil spring which surrounds the heater coil and heat reservoir member which forms one of the parts held together by the aforesaid solder joint. The other part of the solder joint is a relatively rigid, preferably laterally braced conductive support link which extends to one of the'fuse terminals and rigidly supports the entire fuse assembly. The fuse filament extends between the heater coil and the other fuse terminal. The coil spring is held in compression between a shoulder formed on the heat reservoir and a shoulder formed on an in sulating member supportedwithin the fuse housing. With this arrangement, it is apparent no current passes through the spring and the force of the spring is not transferred to i on the heat reservoir member.

the fuse filament. The passage of current through the spring, as in the case with many prior time lag fuses, causes varying degrees of expansion of the spring metal which may obviously affect the low overload blowing characteristics of the fuse.

To simplify the construction and assembly of the time lag fuse of the invention, the contiguous heater coil and fuse filament are formed by diiferent portions of the same piece of wire having the required maximum current carrying capacity to satisfy the high overload blowing characteristics of the fuse. The wire has an insulated coil portion which forms the heater coil surrounding the heat reservoir member and an uncoiled bared end portion extending to one of the fuse terminals in spaced relation to the fuse housing and other parts of the fuse, to minimize heat absorption from this portion of the wire.

' In another form of the invention, the aforesaid conductive support link forms a leaf spring providing a lateral force which tends to separate it from the solder junction Softening or melting of the solder junction enables the support link to pull away from the solder junction thereby breaking the circuit. The heat reservoir member is maintained in a fixed position within the glass body by means including a special resilient support member anchored within the glass tube to be described in detail below.

Other objects, advantages and features of the invention will become apparent upon making reference to the specification to follow, the claims and the drawings wherein:

FIG. 1 is a top view, partially in section, of a time lag fuse constructed in accordance with the present invention;

FIG. 2 is a longitudinal sectional view through the fuse of FIG. 1, taken substantially along the line 22 therein;

FIG. 3 is an exploded perspective view of some of the parts making up the fuse assembly shown in FIGS. 1 and 2 prior to the assembly thereof;

FIG. 4 is an enlarged fragmentary section, throughthe heat reservoir member shown in FIG. 3, taken subs-tam tially along the line 44 therein;

FIG. 5 is a top view, partially in section of the fuse of FIGS. 1-4, when it has been blown by a sustained low overload;

FIG. 6 is a side view, partially in section, of a modified time lag fuse constructed in accordance with the present invention; 1

FIG. 6A is a fragmentary longitudinal sectional view of the plug member forming part of the fuse of FIG. 6 before it has been incorporated in the fuse;

FIG. 7 is a top view, partially in section, of the fuse of FIG. 6;

FIG. 8 is a transverse section through the fuse of FIG. 7, taken substantially along the line 8-8 therein; and

FIG. 9 is an enlarged fragmentary view of the soldered parts of the fuse before being inserted into the fuse housing.

Refer now to the fuse shown in FIGS. 1-5 generally indicated by reference numeral 2. The fuse comprises a housing 4 formed by a cylindrical insulating tube of glass or other suitable insulating material having cupshaped end caps 66 which close off the ends of the tube and form fuse terminals. An assembly of parts 8 constituting a time lag fuse is rigidly supported from one end of the tube 4. The assembly 8 includes a conductive support link It) preferably made from a generally flat strip of a metal alloy chosen for its poor heat conductivity and having a straight outer end portion 12 anchored in a globule 14 of hardened solder adhered to one of the end caps. The metal alloy, for example, may comprise 55 percent copper and 45 percent nickel. The conductive support link is made of a poor heat conductivity alloy to minimize heat loss therethrough which would otherwise be large due to its large mass. The support link 10, of course, is a good conductor of electric current and reference to its conductive properties hereafter applies to its electrical conductivity unless otherwise stated. The solder globule is formed from a body of solder placed within the end cap 6 before its attachment to the end of the tube 4. In attaching the cap to the tube, the end cap is heated and forced over the end of the tube 4. Some of the melted solder is caught between the end cap and tube walls and the rest form a pool of solder in the bottom of the cap.

The conductive support link 10 forms a rigid support for the entire assembly 8. The conductive support link has an intermediate crimped portion 16 which engages the side wall of the tube to brace the assembly against lateral movement accurately to position the same in the tube. The conductive support link has a straight longitudinally extending inner end portion 18 connected to the enlarged outer end portion 20 of a flat copper member 22 forming a heat reservoir member. The heat reservoir member 22 has a relatively narrow shank portion 24 terminating in an enlarged laterally extending inner end portion 26 providing a generally outwardly axially facing shoulder 28.

A heater coil 29 is wrapped around the shank portion 24 of the heat reservoir member, the heater coil comprising a relatively thin body of fuse wire 30 encased in insulation 32 between the end portions thereof. The insulation terminates well short of one end portion 33 of the fuse wire 30. The resulting bared wire end portion 33 extends longitudinally through the glass tube 4 in spaced relation to the walls thereof and to the other parts of the fuse assembly and is embedded within a solder globule 34 adhered to the other end cap 6. The solder globule 34 is formed in the same way as previously described solder globule 14. A body of silicone, plastisol or other insulating filler material 36 fills the end portion of the glass tube 4 immediately adjacent the solder globule 34 to form an air-tight insulation barrier between the adjacent end cap and the heat reservoir member, to prevent the generation of high energy arcs when the fuse is blown by a high overload.

The other end 40 of the fuse wire 30 is bared and wrapped around the narrow shank portion 24 of the heat reservoir member adjacent the enlarged outer end portion 20 thereof and is electrically as well as physically anchored thereto by a solder joint 42. The solder joint 42 also secures the conductive support link 10 to the heat reservoir member and as will appear, forms a time lag fuse body which melts during low sustained overloads. The bared end portion 33 of the fuse wire 30 forms a high overload fuse filament since heat loss from the wire is at a minimum at this portion of the wire which disintegrates under high overload.

The solder joint 42 is formed by momentarily melting a slug 42' of lead solder anchored within an opening 44 (see FIG. 4) of the heat reservoir member 22. The

ends of the solder slug are flattened to form a solder rivet. The projecting ends of the anchored solder rivet enable the bared end 40 of the fuse wire 30 and the conductive support link to be soldered easily to the heat reservoir member in a single operation. The solder carrying end of the heat reservoir member may be held beneath the conductive support link so that one of the projecting ends of the solder slug engages the same. Flux is momentarily applied to the heat reservoir member and the solder iron held against the conductive support link which transfers heat to the solder slug to melt the same. The solder projecting from the then bottom side of the heat reservoir member runs against the bared wrapped end 40 of the fuse wire and the fuse wire and conductive support link are thus simultaneously soldered to the heat reservoir member in a single operation. The voltime and type of solder in the joint 42 determine to a great extent the time lag characteristics of the fuse. The use of an anchored slug of solder of predetermined volume to form the joint enables the obtainment of a consistent predetermined time lag characteristic.

Means are provided for resiliently urging the reser voir member 22 and the conductive support link 10 apart. The means in the embodiment now being described comprises a coil spring 46 which surrounds the assembly of the heater coil 29 and the heat reservoir member .22. The coil spring is sandwiched under compression between the shoulder 28 of the heat reservoir member 22 and a shoulder 48 formed by the reduced end portion 50 of a mica insulating plate 52. The insulating plate 52 has a main rectangular body portion 54 having a width slightly less than the inner dimensions of the glass tube 4 so that the insulating member maintains a fixed aligned position with the glass tube 4. The insulating plate 54 has an angularly' extending slot 56 adapted to receive the strip forming the conductive support link 10. The insulating plate is supported on the support link by positioning the insulating plate thereover with the crimped portion 16 of the support link passing through the base of the opening 56.. The force of the spring 46 and the angle of slot 56 in insulating plate 52 aid in locking the insulating plate on the support link. The coil spring 46 is thus positioned and arranged that no load current flows through the spring and the force of the spring is not applied to the thin bared end portion 33 of the fuse wire 30.

A given sustained low overload for which the fuse is designed to blow will generate sufficient heat to warm .up the heat reservoir member and the solder joint 42 to soften the solder joint to a point where the coil spring 46 pushes the heat reservoir member and heater coil assembly away from the conductive support link 10, as shown in'FIG. 5. The fact that the bared end portion 40 of the fuse wire 30 is wrapped to the very end thereof tightly around the shank portion of the heat reservoir member prevents. the movement thereof away from the heat reservoir member when the solder joint melts, to prevent reclosing of the fuse circuit by a loose end of the fuse wire.

Reference should now be made to the embodiment of the invention shown in FIGS. 6-9. This form of the invention includes the same glass tube fuse housing 4, end caps 66, heat reservoir member 22 and heater coil 29 previously described in connection with the embodiments of FIGS. 1 through 5. The assembly of the heat reservoir member 22 and the heater coil 29 is positioned within the glass tube 4 by means including a generally cylindrical resilient plug member 62 made of silicone rubber or other similar resilient material. The plug member 62 is press fitted within an end portion of the glass tube 4. The plug member 62 has a leg 63 hav- 1ng a support surface 64 which normally inclines radially outwardly toward the end of the leg and upon which bears the assembly of the heat reservoir member 22 and the heater coil 29. This assembly is resiliently urged against the support surface 64 to compress the same into an, axlal plane by spring means 10 which also forms a conductive path to one of the fuse terminals. The spring means thus combines the independent functions of the conduct ve link 10 and the coil spring 46 in FIGS. 1-2. The spring means 10' is formed by a leaf spring having a flat mner end portion 18, an intermediate V-shaped portion 16 joining a transverse end leg 12' located adacent one of the end caps 6 forming the latter fuse terminal. The end leg 12' and part of the intermediate portion 16 is anchored in a hardened globule of solder 14' adhered to the end cap 6.

Thefiat inner end portion 18' of the leaf spring 10' is secured to the enlarged outer end portion 20 of the heat reservoir member 22 by a solder joint 42 formed in the same way as the solder joint 42 in the embodiment of FIGS. 1 through 5. The solder joint 42 also secures one end of the heater coil 29 to the heat reservoir member 22. The leaf spring 10' and the heat reservoir member 22 are so positioned in the glass tube 4 that the leaf spring is under tension which tends to pull it away from the heat reservoir member. The flat inner end portion 18' and the V-shaped intermediate portion l6' of the leaf spring have the angular relationship shown tion of the leaf spring from the solder joint.

in FIG. 9 before it is inserted in the'tube 4 as shown. Then, the spring end portion 18 extends at a sharp angle to the end leg 12'. The V-shaped portion 16' of the spring 10' must be placed under compression to enable the spring to be inserted in the tube. In this position, the leaf spring is braced against opposite sides of the tube 4 and the spring end portion 18' is substantially transverse to the end leg 12.

Under a sustained low overload which softens the solder joint, the leaf spring pulls away from the heat reservoir member 22 and the heater coil 29 to blow the fuse, as shown in dotted lines in FIG. 6. The resilient support surface 64 of the plug member 62 moves the .heat reservoir member and the heater coil mounted thereon in the opposite direction to the leaf spring end 18 (as indicated in dotted lines in FIG. 6'), when the compressive force thereagainst is released by the separa- It is apparent that the leaf spring 10', in addition to its spring forming function, acts as a conductive link which connects the solder joint 42 and the heater coil 29 to one of the end caps 6.

The other bared end 33 of the heater coil fuse wire forming the high overload fuse filament is anchored in solder globular 36' adhered to the other end cap 6. The insulated portion of the heater coil wire next to the bared end 33 of the heater coil wire 30 is sandwiched between the resilient plug member 62 and the walls of the tube 4, which relieves the thin fuse wire 30 of any strain which would tend to break the same. The plug member 62 in addition to forming a positioning and support means for the fuse assembly also acts as an insulating seal or barrier between end cap 6 and the heat reservoir member 22, to prevent the generation of high energy arcs when a high overload disintegrates the fuse filament 33. g

It should be noted that numerous modifications may be made in. the preferred forms of the invention described above without deviating from the broader aspects of the invention.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. A time lag fuse comprising a tube of insulating material forming a fuse housing; a pair of fuse terminals at, opposite ends of said tube; and a fuse assembly in said tube including a pair of resiliently urged apart conductive parts respectively connected to said fuse terminals, a solder joint holding said conductive parts together, one of said conductive parts including a heat reservoir member surrounded by a heater coil, the other conductive part extending to one of said fuse terminals, a high overload fuse filament connected between said heater coil and the other fuse terminal, and means independent of said fuse filament for supporting said fuse assembly rigidly within the tube, said fuse filament being relieved of strains by the independent support provided for t-hefuse assembly.

2. A timelag fuse comprising a tube of insulating material forming a fuse housing; a pair of fuse terminals at opposite ends of said tube; and a fuse assembly in said tube including a pair of resiliently urged apart conductive parts respectively connected to said fuse terminals, a solder joint holding said conductive parts together, one of said conductive parts including a heat reservoir member surrounded by a heater coil, the other conductive part including a conductive link extending to one of said fuse terminals, said conductive link having a crimped portion braced against the side of said .tube to prevent lateral movement of the fuse assembly under normal shock and vibration forces, and a high overload fuse filament connected between said heater coil and the other fuse terminal.

3. The time lag fuse of claim 1 wherein said fuse filament is in laterally spaced relation to said fuse housing and in longitudinally spaced relation to all the other parts of the fuse assembly, and said means for supporting said fuse assembly within said ing: aheat reservoir member within said tube, a heater coilsurrounding said heat reservoir member, a high overload fuse filament electrically connected between one end portion of said heater coil and one of said fuse terminals and being in laterally spaced relation to said fuse housing and in longitudinally spaced relation to all other parts of the fuse assembly, a relatively rigid elongatedconductive support link of substantially greater cross sectional area than said fuse filament, one end of said conductive support link being connected to the other fuse terminal, said conductive support link extending generally longitudinally of said tube and having a laterally extending intermediate portion braced against/the walls of said tube rigidly to support the fuse assembly in a fixed predetermined position in the tube, means electrically connecting the other end of said heater coil to said conductive support link, said last-mentioned means comprising a solder joint securing said conductive link and said heat reservoir member together, means for resiliently urging said heat reservoir member and conductive support link apart, said last-mentioned means comprising a compressed coil spring surrounding said heat reservoir member, an insulating member carried by said conductive link and having a transverse shoulder spaced from the end of said heat reservoir member nearest said one fuse terminal in the direction of the other fuse terminal and against which one end of said coil spring bears, a transverse shoulder on said heat reservoir member adjacent the last-mentioned end thereof and against which the other end of said coil spring bears, said solder joint being adapted to soften under a given sustained low overload to an extent which enables said spring means to separate said conductive link from said adjacent heater coil end and heat reservoir member to open the fuse.

6. A time lag fuse comprising an insulating tube forming a fuse housing, a pair of fuse terminalsat opposite ends of said tube, a resilient conductive link having an inner end connected to one of said fuse terminals, a heat reservoir member comprising a body of metal having lateral extensions at one end forming an axially facing shoulder, an insulated heater coil wound upon said heat reservoir member, a high overload fuse filament, one end portion of said heater coil being connected to the other fuse terminal through said high overload fuse filament, means connecting the other end of said heater coil to said conductive link, said last-mentioned means including a solder joint interconnecting a side of said reservoir member and the outer end of said conductive link, and an insulating member carried by said conductive link and extending longitudinally along said reservoir member, said insulating member having a reduced inner end providing an axially facing shoulder located adjacent the end of said heat reservoir member remote from the end thereof containing said first-mentioned axially facing shoulder, and a compressed coil spring encircling said heat reservoir member and said reduced end portion of said insulating member, the opposite ends of said coil spring being sandwiched between the axially facing shoulders of said heat reservoir and insulating member, said spring forcing said conductive link and reservoir member longitudinally apart when the heat carried by said heat reservoir member to said solder joint under a sustained low overload softens the same.

7. A time lag fuse comprising an insulating tube forming a fuse housing, a pair of fuse terminals at opposite 8 ends of said tube, a resilient conductive link forming a leaf spring connected to one of said fuse terminals, a

heat reservoir member comprising a body of metal having lateral extensions at one end forming an axially facing shoulder, a high overload fuse filament, an insulated heater coil wound upon said heat reservoir member, one end portion of said heater coil being connected to the other fuse terminal through said high overload fuse fila ment, and means connecting the other end of said heater coil to said conductive link, said last-mentioned means including a solder joint interconnecting a side of said reservoir member and a side of said conductive link, said solder joint holding said conductive link under tension under normal load conditions, and the resiliency of said conductive link pulling it away from said solder joint when the heat carried thereto by said heat reservoir member under a sustained low overload softens said solder.

8. A time lag fuse comprising an insulating tube forming a fuse housing, a pair of fuse terminals at opposite ends of said tube, and a fuse assembly in said tube comprising: a heat reservoir member, a heater coil supported upon said heat reservoir member, a high overload fuse filament, one end portion of said heater coil being connected through said high overload fuse filament to one of said fuse terminals, a compressible, resilient support member having a normally inclined support surface, resilient conductive means for connecting the other end of the heater coil to the other fuse terminal, and a solder joint electrically coupling the other end of said heater coil to said conductive means and holding the end of said conductive means under tension upon the side of said heat reservoir member nearest said inclined support surface, said conductive means forcing said heat reservoir member against said inclined support surface to compress the same, said solder joint softening under a sustained low overload, whereupon the resilient force of said conductive means pulls said conductive means laterally away from said heat reservoir member and heater coil, and the resiliency of said inclined support surface moving the heat reservoir member and theheater coil supported thereon away from said conductive means when said solder joint melts under a sustained low overload.

9. A time lag fuse comprising an insulating tube forming a fuse housing, a pair of fuse terminals at opposite ends of said tube, and a fuse assembly in said tube comprising: a heat reservoir member, a heater coil surrounding said heat reservoir member, a high overload fuse filament, one end portion of said heater coil being connected through said high overload fuse filament to one of said fuse terminals, a plug of insulating material in said tube filling said tube adjacent said fuse filament to prevent the generation of an arc between spaced conductive portions of the fuse when the latter disintegrates under high overload, said plug of insulating material having a leg providing a generally axially extending support surface, conductive means connecting the other end of the heater coil to the other fuse terminal, said conductive means including a conductive link forming a leaf spring, and a solder joint electrically coupling the other end of said heater coil to said conductive link and holding the end of said conductive link under tension upon the side of said heat reservoir member nearest said axially extending support surface, said conductive link forcing said heat reservoir member against said support surface, said solder joint softening under a sustained low overload, whereupon the resilient force of said conductive link pulls said conductivelink laterally away from said heat reservoir member and said heater coil.

10. A time lag fuse comprising an insulating tube forming a fuse housing, a pair of fuse terminals at opposite ends of said tube, and a fuse assembly in said tube comprising: a heat reservoir member, a high overload fuse filament, a heater coil supported upon said heat reservoir member, one end portion of said heater coil being connected through said high overload fuse filament to one of said fuse terminals, a support member having a support surface, resilient conductive means connecting the other end of the heater coil to the other fuse terminal, and a solder joint electrically coupling the other end of said heater coil to said conductive means and holding the end of said conductive means under tension upon the side of said heat reservoir member nearest said axially extending support surface, said conductive means resiliently forcing said heat reservoir member against said support surface, said solder joint softening under a sustained low overload, whereupon the resilient force of said conductive means pulls the same laterally aWay from said heat reservoir member and heater coil.

References ite1l in the file of this patent UNITED STATES PATENTS Baldwin et a1 Feb. 3, 1942 Garrison et a1 Mar. 10, 1942 Duerkob Nov. 3, 1942 Duerkob Oct. 2, 1945 Brown Dec. 17, 1946 Yonkers Jan. 3, 1950 Smith Jan. 3, 1950 Berthel Nov. 28, 1950 Von Hoorn Dec. 13, 1955 

1. A TIME LAG FUSE COMPRISING A TUBE OF INSULATING MATERIAL FORMING A FUSE HOUSING; A PAIR OF FUSE TERMINALS AT OPPOSITE ENDS OF SAID TUBE; AND A FUSE ASSEMBLY IN SAID TUBE INCLUDING A PAIR OF RESILIENTLY URGED APART CONDUCTIVE PARTS RESPECTIVELY CONNECTED TO SAID FUSE TERMINALS, A SOLDER JOINT HOLDING SAID CONDUCTIVE PARTS TOGETHER, ONE OF SAID CONDUCTIVE PARTS INCLUDIN G A HEAT RESERVOIR MEMBER SURROUNDED BY A HEATER COIL, THE OTHER CONDUCTIVE PART EXTENDING TO ONE OF SAID FUSE TERMINALS, A HIGH OVERLOAD FUSE FILAMENT CONNECTED BETWEEN SAID HEATER COIL AND THE OTHER FUSE TERMINAL, AND MEANS INDEPENDENT OF SAID FUSE FILAMENT FOR SUPPORTING SAID FUSE ASSEMBLY RIGIDLY WITHIN THE TUBE, SAID FUSE FILAMENT BEING RELIEVED OF STRAINS BY THE INDEPENDENT SUPPORT PROVIDED FOR THE FUSE ASSEMBLY. 